1. Field of the Invention
The present invention relates to a process for preparation of linear organopolysiloxanes having a hydroxyl group at both terminal ends of molecular chain thereof.
2. Description of the Prior Art
Linear organopolysiloxanes having hydroxyl groups at both terminal ends thereof are known to be effective as a dispersing agent in the production of silicone rubber compounds. The performance of this type of organopolysiloxane as dispersing agent increases with the proportion of the hydroxyl groups in the organopolysiloxane. Of the polysiloxanes having hydroxyl groups at both terminal ends thereof, those containing hydroxyl groups in higher proportions, namely, lower-molecular-weight linear organopolysiloxanes with silanol groups at both terminal ends have greater dispersing effects than those containing hydroxyl groups in lower proportions (i.e. higher-molecular-weight polysiloxanes). Therefore, the low-molecular-weight linear organopolysiloxanes have the advantage that, in their use as a dispersing agent for producing a silicone rubber compound, they can be used in a reduced amount, offering another advantage of not impairing processability of the silicone rubber compound.
There have been many studies of methods for preparation of low-molecular-weight linear organopolysiloxanes having hydroxyl groups at both terminal ends thereof. As a preparation method on a laboratory basis, there is known a process in which an alkoxysilane or the like is hydrolyzed in a solution maintained in a neutral condition by use of a buffer or the like. This process, however, is difficult to carry out on an industrial basis. A process in which dimethoxysilane is mixed with an excess of distilled water and refluxed is also known, but according to the process, the yield of the desired product is low.
At present the linear organopolysiloxanes as above-described are produced industrially by subjecting a linear organochlorosiloxane or chlorosilane having chlorine atoms at both its terminal ends to hydrolysis using a weakly alkaline aqueous solution such that cyclic polysiloxanes would not be formed. Due to the instability of the silanol group to acids and alkalis, however, this process involves the problem that HCl formed upon the hydrolysis causes a condensation reaction between the silanol groups, resulting in the formation of higher-molecular-weight organopolysiloxanes and cyclic polysiloxanes in addition to the desired organopolysiloxane. According to the process, therefore, it is very difficult to synthesize a desired low-molecular-weight organopolysiloxane unless neutrality of the reaction system is strictly maintained.
Also known is a process in which the above organochloropolysiloxane is acetoxylated with acetic acid, followed by hydrolysis. However, the process has the drawback that complete hydrolysis is difficult to accomplish and some acetoxyl groups will be left in the reaction product. The organopolysiloxane thus obtained with the residual acetoxyl groups therein is unsuitable for use as a dispersing agent in producing silicone rubber compounds.
U.S. Pat. No. 3,925,285 discloses a process in which a D.sub.3 siloxane (hexamethylcyclotrisiloxane; D stands for a difunctional organosiloxane unit), methanol, formic acid and water are reacted to produce a low-molecular-weight linear polyorganosiloxane having terminal silanol group together with some residual methoxyl groups. This process has the drawback of high cost due to the use of a relatively expensive D.sub.3 type siloxane. In addition, the reactions used in the process do not produce a silanol-terminated low-molecular-weight polyorganosiloxane containing less than three D units. Besides, in view of the reactions, it is impossible to produce a polyorgano-siloxane smaller than 1,5-dihydroxytrisiloxane in chain length. Thus, there is a limitation on the proportion of hydroxyl groups in the reaction product.
U.S. Pat. No. 5,057,620 discloses a process in which a chlorosiloxane (of D.sub.3 type) is added dropwise to a solvent based on an epoxy compound, such as propylene oxide or butylene oxide, and containing water. However, the use of a comparatively expensive D.sub.3 type siloxane is disadvantageous economically, and the low boiling point of the solvent used leads to problems in safety, such as electrostatic ignition.
As the range of silicone rubber applications has expanded more and more, the price of silicone rubbers has been decreasing markedly. It is therefore very important to prepare silicone rubbers at lower cost, and it is highly profitable to synthesize inexpensively a dispersing agent for use in formulating silicone rubber compounds.